An integrated optics device is typically manufactured on a substrate, and comprises a region or regions in which the optical properties of the substrate material have been altered by some means. In particular, very commonly the refractive index of such regions is increased over that of the substrate material, thereby imparting to a properly shaped region the ability to guide electromagnetic radiation of the appropriate wavelength, typically in the visible or near infrared, by means of total internal reflection.
Although some passive devices, e.g., waveguides, directional couplers, and filters, can be manufactured on amorphous substrate materials such as glass, many devices have to be fabricated on monocrystalline substrates, and it is with a subgroup of the latter that this application is concerned.
Materials that have been found to form useful monocrystalline substrates are, inter alia, the ferroelectrics LiNbO.sub.3 and LiTaO.sub.3, which belong to the trigonal crystal class. Trigonal crystals are optically uniaxial, with the crystalline Z-direction conventionally oriented parallel to the optical axis, which is normal to the plane containing the, at least for some purposes nonequivalent, mutually perpendicular X- and Y-directions.
The optical behavior of optically anisotropic crystals such as LiNbO.sub.3 or LiTaO.sub.3 can be described by means of two refractive indices, usually termed the ordinary refractive index n.sub.o and the extraordinary index n.sub.e. Whereas both indices typically are frequency dependent, only n.sub.e depends also on the direction of propagation of the electromagnetic radiation. In the general case, light propagating through an optically anisotropic medium such as an uniaxial crystal is resolved into two nondegenerate modes of well-defined polarization, the isotropic mode, described by the ordinary index of refraction, and the anisotropic mode, whose phase velocity and propagation direction in the general case depend on both n.sub.o and n.sub.e. In integrated optics devices the geometrics typically are arranged such that the guided radiation propagates parallel or nearly parallel to one of the crystal axes. In such a case, both modes always propagate in the same direction, albeit generally at different phase velocities, except when propagating parallel to the Z-direction.
Various methods exist for increasing one or both of the refractive indices of LiNbO.sub.3 or LiTaO.sub.3 which can be used to manufacture waveguiding structures in these materials. Among these are ion implantation, Li out-diffusion, metal in-diffusion, and ion exchange. In particular, in-diffusion of a transition metal, for instance Ti, is currently frequently used for this purpose.
Several of the common methods for raising the refractive index of LiNbO.sub.3 or LiTaO.sub.3, including ion implantation and Ti in-diffusion, affect both n.sub.o and n.sub.e, typically in proportional amounts. Although this is usually a desirable characteristics of these methods, since it, for instance, permits the formation of waveguides capable of guiding modes of either the TE or TM type, the fact that the amount of birefringence of the guiding region is not independently adjustable is a disadvantage for that class of integrated optics devices whose operational characteristics depend on the amount of birefringence of some region of the device. By "amount of birefringence" we mean the difference between the effective indices of a waveguide N.sub.TE and N.sub.TM, if the term refers to a waveguiding region, or to the quantity n.sub.o -n.sub.e, if it does not refer to such a region.
On the other hand, it is known that, for instance, Li out-diffusion as well as Ag/Li ion exchange substantially increase only n.sub.e in LiNbO.sub.3 or LiTaO.sub.3. This property of Li out-diffusion has been used to manufacture waveguides for anisotropic modes in these materials. See for instance J. R. Carruthers et al, Applied Optics, Vol. 13(10) pp. 2333-2342 (1974). Ion exchange has been used previously to manufacture optical waveguides in glass. W. G. French and A. D. Pearson, American Ceramic Society Bulletin, Vol. 49, pp. 974-977, (1970). The technique was later also applied to the manufacture of waveguides in LiNbO.sub.3. N. L. Shah, Applied Physics Letters, Vol. 26(11), pp. 652-653, (1975) immersed polished X-cut samples of LiNbO.sub.3 in a AgNO.sub.3 melt at approximately 260.degree. C., for periods of several hours. It was found that this procedure resulted in the formation of waveguides for the anisotropic (i.e., TE) mode propagating in the Y-direction. Ag/Li exchange thus also substantially increases only n.sub.e in X-cut LiNbO.sub.3. It was also found that this exchange is strongly orientation dependent, proceeding at a substantial rate in X-cut, but not in Y- or Z-cut, crystals.